This invention relates generally to optical signal processing systems and more particularly to beamforming controls for phased array antennas in radar systems.
Phased array antenna systems employ a plurality of individual antenna elements or subarrays of antenna elements that are separately excited to cumulatively produce a transmitted electromagnetic wave that is highly directional. The radiated energy from each of the individual antenna elements or subarrays is of a different phase, respectively, so that an equiphase beamfront, or the cumulative wavefront of electromagnetic energy radiating from all of the antenna elements in the array, travels in a selected direction. The difference in phase or timing between the antenna activating signals determines the direction in which the cumulative beam from all of the individual antenna elements is transmitted. Analysis of the phases of return beams of electromagnetic energy detected by the individual antennas in the array similarly allows determination of the direction from which a return beam arrives.
Beamforming, or the adjustment of the relative phase of the actuating signals for the individual antenna elements (or subarrays of antennas) can be accomplished by electronically shifting the phases of the actuating signals or by introducing a time delay in the different actuating signals that sequentially excite the antenna elements in order to generate the desired direction of beam transmission from the antenna.
Optical control systems are advantageously used to create selected time delays in actuating signals for phased array antenna systems. Such optically-generated time delays are not frequency dependent and thus can be readily applied to broadband phased array antenna systems. For example, optical signals can be processed to establish the selected time delays between individual signals to cause the desired sequential actuation of the transmitting antenna elements, and the optical signals can then be converted to electrical signals, such as by a high speed photodetector array.
Several architectures for optical time delay units have been proposed. For example, an optical beam forming system for a phased array antenna is disclosed in U.S. Pat. No. 5,117,239 of N. Riza entitled "Reversible Time Delay Beamforming Optical Architecture for Phased Array Antennas, " which is assigned to the assignee of the present invention and incorporated herein by reference. These architectures generally depend on the use of linearly polarized light so that light beams of a predetermined polarization are directed through particular paths in the architecture to generate the differential time delay between a delayed and an undelayed signal. Thus, controlling the polarization of a light beam entering the architecture also determines the path that the light beam follows, and the path determines the delay imparted to the light beam.
The optical control system disclosed in the above referenced patent includes a transmit/receive phased array beamformer for generating true-time-delays using optical free-space delay lines and two dimensional liquid crystal spatial light modulators for implementing the optical switching. Unlike conventional optical switching techniques, the liquid crystal-based optical switching elements can provide low insertion loss and low crosstalk level switching with relatively easily fabricated and low cost liquid crystals.
In these polarization based systems using arrays of nematic liquid crystals (NLCs) and polarizing beam splitters to generate the time delay used in controlling the antenna, several factors can cause system performance to be degraded. For example, it is important that the respective light beams be directed through predetermined pixels in each NLC array in the optical architecture so that the polarization of the light beam as it enters each optical delay unit is of the desired orientation in order for the light beam to be directed along the desired path in each optical delay unit. As each light beam must pass through one predetermined liquid crystal (or pixel) in each sequential NLC array, any beam spreading due to free space propagation can result in significant optical losses (or attenuation of the optical signal) and high inter-channel crosstalk (in which the individual light beams spread out so that the light enters other than the desired pixel in each array), both of which reduce the signal to noise ratio in the system. For the same reasons, it is also important that the polarization of each light beam be uniform as it passes through each stage of the optical processing chain.
It is accordingly an object of this invention to provide an optical time delay unit that reduces optical beam spreading in light beams passing through the unit.
It is another object of the present invention to provide an optical time delay unit that maintains a high polarization uniformity in light beams processed in the optical time delay unit.
It is a further object of this invention to provide an optical signal processing system for a phased array antenna system that has low channel crosstalk and a high signal to noise ratio.